An apparatus such as this, a method such as this and a control unit such as this are known, for example, from DE 37 87 355 T2. The apparatus disclosed there is also illustrated in FIG. 1 of the attached drawing. The already known apparatus 1 has a control coil 2 and an uncontrolled coil 6 which can be inserted in series into a high-voltage line, which carries alternating current, by means of expedient connections or switches which are not shown in FIG. 1. In order to control the effective impedance of the control coil 2 in the apparatus 8, a switching device 3 is provided, and is formed by two thyristors which are connected in opposite senses. The thyristors are connected to a control unit 4, whose control signals allow the current flow through the thyristors to be interrupted or permitted. The effective impedance of the control coil 2 in the apparatus, and thus the impedance of the entire apparatus, can be adjusted by expedient choice of the triggering time of the thyristors as a function of the phase of an alternating current in the high-voltage line. This response time of the thyristors, which is dependent on the phase of the alternating current, is referred to in the following text as the trigger angle. The use of the thyristors in the control path 5 for solid-state interruption of the alternating current also results in higher-order harmonic of the fundamental frequency of the alternating current, that is to say integer multiples of the fundamental frequency of the alternating current. For this reason, a filter unit 7 is provided in parallel with the control path 5 and is designed to suppress one or more of these harmonic oscillation components. Furthermore, the filter unit and the filter coil 6 are used to maintain the current path through the apparatus when the switching device 3 is in a position in which the current flow is interrupted. However, the already known apparatus occupies a large amount of space and is costly because of these additional components. Furthermore, the control capabilities of the already known apparatus are restricted, since its impedance cannot be reduced to zero, but only to the impedance of the parallel-connected coils 2 and 6.